How much should I bevel?
It might not be a sexy answer, but I'd say: measure. If you want to simulate the real world, know your world.

What do you do if you want to model a realistic excavator? You either get a good model or you use reference picture. You imitate the real thing which has the knowledge of engineers. You use their knowledge. Getting bevel sizes is doing the same thing in detail.

It's mostly not shape or size, it's the manufacturing process or the intended use that decide on the roundness of anygiven edge. 

So, get into the real world and measure bevels. And make a reference chart.

When I wanted to model a typewriter, I decided to measure all the bevels to get the proportions right.

The beauty of 3D is that you learn much more than to simply use the software.